In general, an electric motor is called a motor, and is a device that converts electrical energy into mechanical energy by using power received by a conductor in which current flows in a magnetic field. The electric motor typically includes a cylindrical stator that is fixed into a frame and around which a coil is wound, and a rotor that is disposed inside or outside the stator and transfers a power while being rotated by induced magnetism formed near the coil.
The electric motor may be classified as a DC electric motor, an induction electric motor, a synchronous electric motor, and an AC commutator electric motor according to rotation principles.
Loss is necessarily generated due to a structural matter when the electric motor is driven. Copper loss and iron loss are the main losses generated in the induction electric motor and a permanent magnet synchronous electric motor.
In order to increase driving efficiency of the electric motor, the copper loss and the iron loss need to be appropriately reduced. Since the iron loss of the electric motor is usually generated in a stator core, rather than a rotor core, it is most important to measure and reduce the iron loss of the stator core to thereby increase efficiency of the electric motor.
The iron loss generated in the motor stator core is characterized by a waveform of applied magnetic flux density and an intrinsic loss characteristic of an electric steel sheet (e.g., a silicon steel sheet) used to manufacture the stator core.
The motor stator core is generally manufactured through a punching process, a laminating process, a winding process, a varnishing process, and a housing inserting process. Since mechanical stress is applied to the motor stator core in a series of manufacturing processes, the intrinsic loss characteristic of the electric steel sheet, which is a material of the motor stator core, is changed after the manufacturing processes, and the iron loss is generally increased for the same magnetic flux density waveform.
FIG. 1 illustrates a change in iron loss for each of the various processes of manufacturing a motor stator core. The sample is an isotropic electrical steel sheet, and a measurement frequency is 200 Hz. The iron loss of the motor stator core is gradually increased for each of the manufacturing processes.
Accordingly, in order to reduce the iron loss generated in the motor stator core, each manufacturing process needs to be managed so as not to excessively increase iron loss by quantitatively measuring the iron loss generated for each of processes of manufacturing the motor stator core.
In order to measure the iron loss of the motor stator core for each of the manufacturing processes described above, there is disclosed a measuring method using a C-shaped auxiliary core in the related art.
As illustrated in FIG. 2, the motor stator core used for measuring the iron loss has a structure in which a stator yoke 10 and stator teeth 20 are formed as one iron core. A plurality of stator teeth 20 is formed on an inner circumferential surface of the stator yoke 10 spaced-apart in a circumferential direction, and slots 30 are formed between the stator teeth 20. A coil is inserted into the slots 30 to be wound around the stator teeth 20.
One or more fixing grooves 40 that are concave inward in a radial direction are formed on an outer circumferential surface of the motor stator core spaced-apart in the circumferential direction. The fixing grooves 40 serve to fix the stator core to an inner wall of a housing when the stator core is inserted and provided into the housing.
A C-shaped auxiliary core 50 is provided in an inner space of the stator core used for measuring the iron loss of the stator core. The C-shaped auxiliary core 50 is provided to come in contact with two stator teeth 20, which are spaced-apart from each other in the circumferential direction, among the plurality of stator teeth 20. The C-shaped auxiliary core 50 is provided such that two ends thereof come in contact with inner ends of the two stator teeth 20, which are spaced-apart from each other in the circumferential direction, in the radial direction. Two excitation windings 52 are wound around certain portions of the C-shaped auxiliary core 50 in a longitudinal direction, and the two excitation windings 52 are disposed to be spaced-apart from each other in the longitudinal direction of the C-shaped auxiliary core 50.
Further, a B-coil 54 is wound around a portion positioned inside any one excitation winding 52 of the two excitation windings 52 at a certain portion of the C-shaped auxiliary core 50 in the longitudinal direction. The B-coil 54 is a sensor coil used to measure an induced voltage.
After the C-shaped auxiliary core 50 is provided at the stator core, magnetic flux density, magnetic field intensity, and iron loss of the stator core are calculated using the C-shaped auxiliary core 50 from the following equation.
Magnetic flux density H(t)=N*i(t)/L (where N is the number of excitation windings, i(t) is the excitation current, and L is the useful magnetic flux path);
Magnetic field intensity
      B    ⁡          (      t      )        =            -              1        NA              ×          ∫                        e          ⁡                      (            t            )                          ⁢                  ⅆ          t                    (where N is the number of winding of sensor coil, A is the cross-section area of C-shaped auxiliary core, and e(t) is the induced voltage); and
Iron loss
  P  =            1      T        ⁢          ∫                        H          ⁡                      (            t            )                          ⁢                              ⅆ                          B              ⁡                              (                t                )                                                          ⅆ            t                          ⁢                  ⅆ          t                    (where T is the cycle).
That is, the magnetic flux density is calculated using excitation current, the magnetic field intensity is calculated by measuring a voltage generated in the sensor coil (B-coil) of the C-shaped auxiliary core, and the iron loss is calculated by subtracting copper loss of the excitation winding from an input power.
As illustrated in FIG. 3, the method for measuring the iron loss of the motor stator core using the C-shaped auxiliary core of the related art can quantitatively measure a change in iron loss generated after a punching process of the stator core, a laminating process, a winding process 60, a varnishing process, and an inserting process of housing 70.
However, in the method for measuring the iron loss of the motor stator core using the C-shaped auxiliary core of the related art, in order to improve measurement accuracy, end surfaces of the C-shaped auxiliary core 50 need to come in close contact with inner end surfaces of the stator teeth 20 in the radial direction, as illustrated in FIG. 4.
When the end surfaces of the C-shaped auxiliary core 50 do not come in close contact with the inner end surfaces of the stator teeth 20 in the radial direction, such as when a gap is generated therebetween, vibration caused by electromagnetic force between two cores reduces the measurement accuracy.
However, due to tolerance when the motor stator core is manufactured, or an angle error generated when the C-shaped auxiliary core is inserted and attached to the housing 70, the end surfaces of the C-shaped auxiliary core and the end surfaces of the stator teeth 20 may not completely come in contact with each other, as illustrated in FIG. 5.
When iron loss is measured in a state where the end surfaces of the C-shaped auxiliary core and the end surfaces of the stator teeth 20 do not completely come in contact with each other, electromagnetic force may be alternately generated between the stator teeth 20 and the C-shaped auxiliary core 50 to cause vibration in the C-shaped auxiliary core 50, and the end surfaces of the stator teeth 20 and the end surfaces of the C-shaped auxiliary core 50 may be misaligned due to the vibration. As a result, the C-shaped auxiliary core 50 is rotated by a predetermined angle in the circumferential direction as illustrated in FIG. 6, and, thus, the end surfaces of the C-shaped auxiliary core 50 may be slung between the two stator teeth 20.
Further, a gap may be formed between two cores due to the vibration in the C-shaped auxiliary core 50, and an error may be caused in measuring magnetic field intensity due to additional magnetomotive force consumed in the gap. In addition, a size of a cross-section in which magnetic flux flows is changed due to the rotation of the C-shaped auxiliary core 50, and, thus, the measurement accuracy may be degraded.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.